FIG. 7 illustrates a conventional circuit board (refer to the following patent document 1). The illustrated circuit board X is used in e.g. battery pack for a mobile telephone. In this case, the circuit board X is provided with a protection circuit for preventing over discharge and overcharge of a rechargeable battery incorporated in the battery pack. As shown in the figure, the circuit board X includes an insulating base board 91 and a pair of rectangular metal plates 93 provided on the base board 91. The base board 91 is provided with a plurality of electronic components 92. Though not shown, a wiring pattern is formed on the base board 91 and connects the electronic components 92 and the metal plates 93 to each other.
As shown in FIG. 7, each of the metal plates 93 is welded to a conductive strip 95. Specifically, one end of the conductive strip 95 is welded to the metal plate 93, and the rest laterally protrudes beyond the base board 91. This protruding portion is bent into a predetermined form to be used as a connecting terminal relative to the rechargeable battery. As shown in FIG. 8, the welding process of the conductive strip 95 may be performed utilizing a welding device Y having two welding rods 97 (refer to the following patent document 2).
Patent Document 1: JP-A-2002-208788
Patent Document 2: JP-A-2002-144047
As shown in FIG. 9, the metal plate 93 is bonded to a pad 94a, which is a part of the wiring pattern, via a solder layer 96. The solder layer 96 covers entire under surface of the metal plate 93. Due to this structure, following problem occurs.
As shown in FIG. 9, in welding the conductive strip 95 to the metal plate 93, the welding rod 97 is pressed onto the conductive strip 95, so that the conductive strip 95 contacts the metal plate 93 intimately. In this state, an electric current is applied to the strip 95 and to the metal plate 93 through the welding rod 97. Due to heat caused by the electric current, the strip 95 is welded to the metal plate 93. This technique is commonly known as “resistance welding”.
According to the conventional structure, the heat in welding tends to be transferred to the solder layer 96 via the metal plate 93, and may melt the solder layer 96. Further, depending on the welding condition (such as the amount of pressing force of the welding rod 97 or the amount of electric current to be applied), the melted solder may be scattered around the metal plate 93. If this scattered solder sticks to the electronic components 92, these components may not work normally, or may be damaged. To solve this problem, the electronic components 92 may be sealed by resin. However, if such structure is used, the number of manufacturing process or components is increased, and thus the product cost is increased.